High dynamic range (“HDR”), and/or high contrast, projectors attempt to maximize contrast between black portions of projected images and white portions of projected images. Some HDR projectors are based on digital micromirror devices (“DMD”, such as Digital Light Processing or DLP™), and the high dynamic range is achieved with two stages of imaging: a first pre-modulator DMD that creates an initial “normal” contrast version of an image and a second prime imaging DMD that re-images the initial “normal” contrast version of the image from the first DMD, a second time (which can be at a higher resolution), to create blacker blacks in the final image and thus increasing the final image contrast.
Existing architectures of such HDR projectors use the first DMD in a reverse configuration and the second DMD in a forward configuration. In the forward configuration, which is the configuration in which most DMDs are designed to operate, a DMD is illuminated at an angle to a plane of the DMD and an image formed by the DMD is reflected normal (perpendicular) to the plane of the DMD. However, in the reverse configuration, a plane of the DMD is illuminated perpendicularly, and an image formed by the DMD is reflected at an angle to the plane.
Operating the first DMD in the reverse configuration in HDR projectors has been necessary when using traditional projection optics because of the Scheimpflug principle, which describes under what conditions it is possible to focus two optical planes onto each other to achieve acceptable focus of light from the first DMD onto the second DMD. Indeed, if the first DMD were operated in the forward configuration using traditional optics, a subject plane of the first DMD could not be focussed on the second DMD, other than in a narrow region. As a result, the first DMD in existing-architecture HDR projectors is operated in the reverse configuration. However, operating the first DMD in the reverse configuration results in a system that is significantly less optically efficient, due to diffractive effects of the DMD mirror array, which acts as a blaze grating, and which can lead to increases in projector cost, size, noise and power consumption.